We have reported a cDNA isolated by differential hybridization between differentiated and undifferentiated colon carcinoma cells. The novel transcript, cori-1, is complementary to the 5' untranslated region of the krev-1 anti-oncogene mRNA. Cori-1 mRNA has an open reading frame which encodes a protein identical with the rat S29 ribosomal protein. This protein has a potential zinc finger motif consisting of four cysteine residues. In order to examine the biological function of the protein, mutant cori-1 proteins synthesized in E. coli were fixed on a membrane and incubated with 55Zn after denaturation. Only the wild type was labeled with 55Zn, indicating that this form of the protein, but not the mutant form, may take a zinc finger conformation. The wild type and mutant cori-1 genes were inserted into an expression vector and transfected into HT1080 human fibrosarcoma cells. In HT1080 cells transfected with mutant cori-1, both in vitro and in vivo cell growth was enhanced compared with the cells transfected with vector only. On the other hand, cell growth both in vitro and in vivo was not enhanced when the wild type cori-1 was introduced. The expression of krev-1 as determined by either mRNA or protein was not changed by introducing either wild type or mutant cori-1 into HT1080 cells. These results suggest that the trans-dominant suppression of endogenous cori-1 by the mutant form of the gene enhanced cell growth and the tumorigenic phenotype. This function of the mutant cori-1 is conferred by interfering with a function dependent on the zinc finger domain without affecting krev-1 expression.